In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides.@en

In this work, the different techniques for non-destructive in situ measurement of chloride ion concentration are presented. Non-destructive (ND) in situ measurement is crucial for reliable and continuous determination of chloride ion concentration in concrete. Over the last 20 years, several studies have been performed on ND measurements. These were mainly focused on the application of electrochemical and electromagnetic techniques. Each technique has its advantages and disadvantages. Depending on the requirement of assets managers and constructors and considering the limitations, these techniques can be well applied. The main concepts and comparative analysis, in view of possibilities and limitations, of these non-destructive techniques are presented in this paper.@en

The stability and reproducibility of an Ag/AgCl sensors’ response in an alkaline medium are important for the application of these sensors in cementitious materials. The sensors’ response, or their open circuit potential (OCP), reflects a dynamic equilibrium at the sensor/environment interface. The OCP response in an alkaline medium is affected by the presence of hydroxide ions. The interference of hydroxide ions leads to inaccuracies or a delay in the sensors’ response to a certain chloride content. In this article, the potentiometric response (or OCP evolution) of the chloride sensors is measured in model solutions, resembling the concrete pore water. The scatter of the sensors’ OCP is discussed with respect to the interference of hydroxide ions at varying chloride concentration in the medium. The deviation of the sensor’s response from its ideal performance (determined by the Nernst law) is attributed to dechlorination of the AgCl layer and the formation of Ag2O on the sensor’s surface. Results from the surface XPS analysis of the AgCl layer before and after treatment in alkaline medium confirm these observations in view of chemical transformation of AgCl to Ag2O.@en

Determination of the free chloride content in a concrete structure is important for evaluation of the risk of corrosion of reinforcing steel. The Ag/AgCl sensor is sensitive to chloride ions and exhibits an open circuit potential (OCP) that depends on the chloride ions activity in the environment. The interference of hydroxide and sulfide ions in the pore solution of cementitious materials with the sensor affects the stability of the sensor’s OCP. In this paper, the sensor’s OCP (i.e. sensor reading) in Portland and slag cement pastes is monitored over 300 days of immersion in solutions with different chloride concentration. The results show the applicability of the chloride sensor for continuous and non-destructive determination of the free chloride content in Portland cement paste. The significantly different OCP of the sensor in slag cement paste was attributed to the interference of sulfide ions and formation of Ag2S on the sensor’s surface. Hence, the content of free chloride ions in slag cement paste could not be determined from the sensor’s OCP. The acid-soluble chloride and water-soluble chloride in the cement pastes were also determined after 300 days of immersion of specimens in the solutions. In case of Portland cement paste, the measured acid and water-soluble chlorides were compared to the sensor reading. The acid and water-soluble chlorides were higher than sensor reading. The release of physically and chemically bound chlorides into the acid and water solvents were the main cause for this trend. The results show the importance of cement type and chloride binding ability of hydration products for the sensor’s response and the acid and water-soluble chloride contents. @en

Determination of the chloride content in a reinforced concrete structure is important for evaluation of the risk of chloride-induced corrosion of reinforcement. The traditional techniques for chloride determination in concrete are laborious, time-consuming and cannot be used for continuous monitoring of the chloride content. The investigation on the use of Ag/AgCl electrodes as chloride sensors in cement-based materials dates back to 1990s. Interpretation of the sensor’s response in cementitious materials requires the knowledge of chloride sensor’s characteristics and the interaction between the sensor and the surrounding medium. Hence, the stability of the chloride sensor’s response in cementitious materials depends on the properties of Ag/AgCl interface, AgCl/cement paste interface and the pore solution composition of cementitious materials. The influence of these factors on the stability of the sensor’s response was studied in this thesis. In Chapter 1 the background and motivation for the thesis were presented. In Chapter 2 the advantages and drawbacks of available test methods for determination of the chloride content in cementitious materials were explained. @en

Characterization of the Ag/AgCl electrode is a necessary step toward its
application as a chloride sensor in a highly alkaline medium, such as concrete.
The nucleation and growth of AgCl on Ag in 0.1 M HCl was verified through
cyclic voltammetry. Ag anodization was performed at current densities, determined
by potentiodynamic polarization in the same (0.1 M HCl) medium. The
morphology and microstructure of the AgCl layers were evaluated via electron
microscopy, while surface chemistry was studied through energy-dispersive
spectroscopy and X-ray photoelectron spectroscopy. At current density above
2 mA/cm2, the thickness and heterogeneity of the AgCl layer increased. In this
condition, small AgCl particles formed in the immediate vicinity of the Ag
substrate, subsequently weakening the bond strength of the Ag/AgCl interface.
Silver oxide-based or carbon-based impurities were present on the surface of the
sensor in amounts proportional to the thickness and heterogeneity of the AgCl
layer. It is concluded that a well-defined link exists between the properties of the
AgCl layer, the applied current density and the recorded overpotential during
Ag anodization. The results can be used as a recommendation for preparation of
chloride sensors with stable performance in cementitious materials.@en

This paper reports on the advantages and drawbacks of available test methods for the determination of chloride content in cementitious materials in general, and the application of Ag/AgCl chloride sensors in particular. The main factors that affect the reliability of a chloride sensor are presented. The thermodynamic behaviour of silver in the presence or absence of chloride ions is described and kinetic restrictions are addressed. The parameters that can affect the activity of chloride ions in the medium and/or the rate of ion exchange and dissolution/precipitation processes at the sensor’s surface are also considered. In this regard, the contribution of morphology and microstructure of the AgCl layer, binding of chloride ions and the compactness of hydration products around the chloride sensor are highlighted. The important parameters for a reliable sensor’s response are discussed and the possible causes of inaccuracies are evaluated@en

When studying chloride-induced corrosion in reinforced concrete structures, essential information of interest is the concentration of chloride ions in the system. The absence of a reliable method for monitoring the free chloride ions justifies the attempts towards establishing a feasible practice in the application of the already known Ag/AgCl electrode, as a chloride sensor. To identify the governing mechanism and cognition of causes for instability of the chloride sensors in highly alkaline medium (as concrete), it is necessary to study the polarization behaviour of silver in different aqueous solutions resembling the concrete environment. Following expectations and well-known fundamental background, the results from this work confirm that in the presence of chloride ions, silver chloride is the predominant reaction product, forming on the silver surface. Whereas, in the absence of chloride ions and/or presence of interfering ions, such as hydroxide ions, the oxidation process of AgCl formation is significantly dependent on the chloride concentration in the medium. Therefore, the formation of a stable AgCl layer on a Ag substrate (as would be required for sensors application for example) is a function of the presence and amount of interfering ions, together with the chloride concentration in the medium.@en

The need for an accurate determination of the chloride threshold value for corrosion initiation in reinforced concrete has long been recognized. Numerous investigations and reports on this subject are available. However, the obtained chloride threshold values have always been, and still are, debatable. The main concern is linked to the methods for corrosion detection and chloride content determination in view of the critical chloride content itself. In order to measure the chloride content, relevant to the corrosion initiation on steel, destructive methods are used. These traditional methods are inaccurate, expensive, time consuming and noncontinuous. Therefore, the application of a cost-effective Ag/AgCl ion selective electrode (chloride sensor) to measure the chloride content directly and continuously is desirable. The advantage would be an in situ measurement, in depth of the concrete bulk, as well as at the steel/concrete interface.

The aim of this work was to evaluate the importance of the sensor’s properties for a reliable chloride content measurement. The main point of interest with this regard was the contribution of the AgCl layer and Ag/AgCl interface within the process of chloride content determination in cementitious materials. The electrochemical behavior of sensors and steel, both embedded in cement paste in a close proximity, hence in identical environment, were recorded and outcomes correlated towards clarifying the objectives of this work. The main point of interest was to simultaneously detect and correlate the time to corrosion initiation and the critical chloride content.

The electrochemical response of steel was monitored to determine the onset of corrosion activity, whereas the sensors’ electrochemical response accounted for the chloride content. For evaluating the electrochemical state of both sensors and steel, electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements were employed. The results confirm that determination of the time to corrosion initiation is not always possible and straightforward through the application of OCP tests only. In contrast, EIS is a nondestructive and reliable method for determination of corrosion activity over time. The obtained results for corrosion current densities for the embedded steel, determined by EIS, were in a good agreement with the sensors’ half-cell potential readings. In other words, the sensors are able to accurately determine the chloride ions activity at the steel/cement paste interface, which in turn brings about detectable by EIS changes in the active/passive state of steel.

The electrochemical response was supported by studies on the morphology and surface chemistry of the sensors, derived from electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS). It can be concluded that the accuracy of the sensors, within detection of the time to corrosion initiation and critical chloride content, is determined by the sensors’ properties in terms of thickness and morphology of the AgCl layer, being an integral part of the Ag/AgCl sensors.
@en

Free chloride ions are the main cause of steel corrosion in reinforced concrete structures. Ag/AgCl electrodes, as chloride sensors, are predominantly sensitive to chloride ions and respond electrochemically to the chloride ion activity in the environment. This is reflected by changes of the open circuit potential (OCP) of the sensors. To investigate the effect of the cement-based matrix on OCP readings, this paper presents the electrochemical responses of chloride sensors, embedded in cement paste specimens of hydrated pure cement phases of different water-to-binder ratios. The specimens were immersed in alkaline solutions with various chloride concentrations. The results show dependency of the OCP of the sensor on the surrounding hydrated cement-based matrix. This effect was more pronounced when the specimens were immersed in solutions of lower chloride concentration. In contrast, these effects were negligible at high (> 500 mM) chloride concentrations@en

Service life of reinforced concrete structures exposed to chloride enriched environment is well known to be mainly determined by steel passivity breakdown in the event of chloride-induced corrosion initiation and propagation. Since Cl- induced (localized) corrosion is caused by the free (water-soluble) chloride, present in the pore network of a reinforced concrete structure, quantifying the level of free chloride locally, e.g. via embedded Ag/AgCl electrodes (chloride sensors), is a generally accepted and applicable approach to monitor the time to corrosion initiation. The measurement is essentially a potential (voltage) reading over time and as such logically depends, among other factors, on the electrical and microstructural properties of the surrounding medium. Therefore, an accurate determination of the time to corrosion initiation significantly depends on the properties of relevant interfaces, such as the steel|cement paste interface and/or the Ag|AgCl|cement paste interface. In this paper, steel rods were coupled with Ag/AgCl sensors and embedded in cement paste cylinders. The specimens were immersed in simulated pore solution, containing 855 mM chloride concentration. Electrochemical impedance spectroscopy (EIS) was employed for quali-/quantification of the corrosion process on the steel surface (medium to low frequency response), while simultaneously providing information for the electrical properties of the bulk cement-based matrix through the high frequency response. The open circuit potential (OCP) values of both sensors and steel rods were recorded for more than 30 days. The results show a good agreement between sensor readings and steel electrochemical response i.e. time to corrosion initiation was recorded via steel OCP readings, whereas the relevant chloride content was estimated via the sensors’ OCP readings. @en

The half-cell potential of the Activated carbon (AC), due to its high double layer capacitance (EDL), remains stable in high ionic electrolyte. The open circuit potential (OCV) of the AC, with EDL of 40 – 50 F, shows a stable potential (10 mV variation) over two weeks in the cement pore solution (sat. Ca(OH)2). The OCV is less sensitive to the pH change from pH 8 to 12 (range inside concrete). The potentiometric response of a Ag/AgCl vs. AC pseudo-reference shows a near-Nernstian response (-51 mV/decade). The AC electrode can be proposed as a pseudo-reference for the potentiometric monitoring inside concrete.@en

The stability of chloride ion selective electrodes in highly alkaline solutions is an important aspect when reliability and feasibility within reinforced concrete applications are concerned. The sensitivity of these electrodes largely depends on the properties of the AgCl layer, including uniformity, porosity, orientation, thickness, etc. These can be varied for optimum performance through adjusting the current density and anodizing time. In this study the Ag|AgCl electrodes were prepared in four different current density regimes and the resulting microstructural properties of the AgCl layer were correlated to potentiometric response in chloride-containing solutions, including synthetic concrete pore solution and cement extract. It was found out that lower level of the current density within the anodization process results later on in increased sensitivity and stability of the electrodes..@en